DESCRIPTION: The PI plans to continue his very successful research program on the genetic basis of speciation. This Drosophila based research has primarily focused on differences between very closely related species that may have been important in the isolation of species. In particular, much of the proposed work is motivated by a growing body of evidence that pre- zygotic isolation may be the most common and first arising form of isolation for speciation events. The study of phenotypes associated with recent speciation events can be complex. One class of phenotypes are species differences that may have arisen as a component of the evolution of reproductive isolation. Often these differences are sex-limited so that there are at least four classes of organisms to examine in this context (males and females in each of two species). A second class of phenotype are features of pairs of organisms (e.g. anything associated with pre- zygotic limits to hybrid sex, such as mate choice) and there are two reciprocal pairs that can be considered for each pair of sibling species. Also these phenotypes must usually be measured in comparison to their intra-specific form. A third class includes at least two interesting phenotypes that are only manifest in species hybrids, sterility and inviability. These can be studied in both reciprocal crosses, potentially, depending on hybrid inviability and the genetic tools available to avoid it. The proposed research will focus on phenotypes in each of these three categories, subjecting each to a gene mapping protocol.